Summary It is widely recognized that vegetation restoration will have positive effects on watershed health by reducing soil erosion and non-point source pollution, enhancing terrestrial and aquatic habitat, and increasing ecosystem carbon sequestration. However, the hydrologic consequences of forestation on degraded lands are not well studied in the forest hydrology community as a whole. China has the largest area of forest plantations in the world now, and the hydrologic consequences of massive forestation are unknown. We applied a simplified hydrological model across the diverse physiographic region to estimate the potential magnitude of annual water yield response to forestation. Our study suggests that the average water yield reduction may vary from about 50 mm/yr (50%) in the semi-arid Loess Plateau region in northern China to about 300 mm/yr (30%) in the tropical southern region. We conclude that forestation in China that often involves a combination of tree planting and engineering (e.g., terracing) may have even a higher potential to greatly reduce annual water yield in headwater watersheds, especially in the semi-arid Loess Plateau region. However, the forestation area is relatively small for most large basins with mixed landuses in China, thus the regional effects of forestation on water resource management may not be of major concern. Comprehensive science-based evaluation of roles of forests on regulating regional water resources is critical to the current forestation endeavors in China.